Conventionally, it is necessary before most of the analyzing treatments of compounds such as abovementioned, to proceed with a chemical reaction, for example a mineralization by wet process using concentrated acids, such as sulfuric acid, nitric acid, perchloric acid, or mixtures thereof, or even a saponification by alcoholic potash, an oxidation or a reduction.
To carry out such a prior analysis treatment successfully, a conventional method consists in placing in a receptacle, such as of flask type, a quantity of the compound to be analyzed, as well as the necessary complementary volume of specific concentrated reagent.
The container is then heated and permanently watched by an operator whose responsibility it is to periodically control the calories transmitted to the receptacle in order to avoid the appearance of foams and overflowing of the product. The operator must also, at frequent intervals, stir the mixture being heated, in order to preserve a good homogeneity of the dissolved and heated compound. Finally, it is also common for reagents to be introduced in the course of the treatment, either to replace the reagent that has evaporated, or to introduce other reagents for speeding up the reaction.
Such vigilance on the part of the operator is tedious and time-consuming, moreover it is liable to provoke injuries that could be serious to the person of the operator, through risks of explosion, spattering of the composition and through the emanation of corrosive fumes and vapors.
In addition to the above drawbacks, it should be noted that the method applied does not make it possible to control efficiently any departure of fractions of the products included in the basic composition, which are driven away by the fumes and vapors escaping through the neck of the container. Consequently, the subsequent analysis result is considerably disturbed, and there is no control means which can permit an assessment of the existence and importance of this random negative factor.
In an attempt to perfect such a process, various improvements have been developed and implemented.
For example, installations have been proposed which use as heating means, instead of a direct flame, a bank of crucibles with infrared radiation, the power of each crucible being adjustable by control of its own electrical supply. Such an installation however, does not solve the problems of permanent surveillance, of stirring and of doubtful results, given that these three factors are always dependent on the permanent supervision of one or more operators.
Still with a view to improving such a process for prior analysis treatment, it has been proposed to use, as heat generator, a microwave oven provided with a fume and vapor suction port.
U.S. Pat. No. 4 090 168 teaches such a proposition and recommends the use of a microwave oven in whose cavity is placed a flask containing the sample to be treated.
This technique offers definite advantages in that the application of microwaves allows a quasi-uniform distribution of energy hence a thorough heating of the compound to be treated. For the same reasons, it is possible with such a technique to break the foams which develop with the preceding technique when the temperature of the compound rises. Therefore, theoretically, a permanent surveillance of the evolution of the treatment is no longer necessary, thus releasing the operator from this tedious and dangerous aspect of the prior method.
Moreover, the microwave oven is known for its properties of having no thermal inertia, which permits a more accurate control of the energy transmitted to the sample.
It has however been found that such an apparatus was not entirely satisfactory. Indeed, it has been observed that a certain number of products could not be treated due, in particular, to the rapid ebullition of the added reagent.
It has also been found that subsequent analysis results were not always accurate and that they implicated a factor of doubt which is no inconsiderable for certain products and is even prohibitory for others. More precisely, a loss of certain products was noted, caused by considerable amounts of fumes and vapors driving away fractions of these products, through selective evaporation and/or priming effect.
Because of the strong evaporation, a considerable pollution of the cavity of application has also been noted, necessitating cleaning of the cavity walls after each reaction and before proceeding with the subsequent treatment of a sample of same nature or not.
Another negative result of that strong evaporation is that a vapor fog is created inside the cavity where the receptacle is placed, preventing any visual assessment of the evolution of the reaction underway.
Consequently, the technique recommended by this U.S. Patent cannot, in practice, be used at the industrial level. This fact has actually been realized for some years already, since the publication by BARRETT, DAVIDOWSKI, PENARO, COPELAND, Anal. Chem. 1978-50, 1021 has disclosed an improved technique derived from the teaching of said U.S. patent, whose object is to reduce, without eliminating them, the drawbacks caused by the strong evaporation.
To improve considerably the mineralization technique making use of microwave energy, French Patent Application No. 84 03498 proposes a chemical reaction apparatus comprising a glass receptacle defining a retention volume extended by a substantially cylindrical neck. Such an apparatus further comprises a microwave energy application cavity of height substantially equal to the height of the receptacle's container portion, provided at its top with a permanently open opening of section equal, to within the clearance, to the largest measurement of the container portion taken perpendicularly to the axis of the neck, and having a stack bordering the opening and rising over a sufficient height as a function of the section of passage of the opening in order to form an absorption barrier preventing the propagation of the microwaves outside the cavity.
It is undeniable that this proposition has considerably improved the prior art by providing the means of carrying out a chemical reaction on an individual sample, without any risk of causing severe polution to the cavity.
Moreover, such a technique has made it possible to control with more accuracy than reflux rate, in order to obtain a more accurate reaction assessment.
The apparatus which have been built on the above basis have, however, proved limited as regard practical implementation, and this for the following reasons.
Said apparatus were initially designed so as to allow individual heating of flask-type receptacles with a serviceable capacity of 15 ml, capable of containing a mass of about 250 mg of inorganic, organic or organometallic compounds to be treated. It turns out that, to improve mineralization accuracy and to go along with current treatment trends, a serviceable capacity of 30 ml offering a retention capacity of 0.5 to 10 g of compounds would be more suitable.
When applying the technique of the above French Patent to carry out a Kjeldahl type mineralization, it would seem important, if not imperative, to have the use of a total capacity of at least 250 ml, in order to avoid certain transfer operations after mineralization.
The apparatus implemented in the above French Patent have also proved, with wear, to be delicate, because of the necessity of introducing through the stack every receptacle which then run the risk of being broken. Such a risk also exists from thermal shock.
Broken pieces of receptacles or flasks were found after the manipulations which had to take place, as well as dispersion or spattering of the compound to be treated, inside the application cavity.
For various understandable reasons, such a risk should be reduced to a minimum, if not eliminated altogether in order for the practical use to meet industrial requirements.
In other words, the apparatus according to the abovementioned French Patent Application has not proved entirely satisfactory, principally because of the capacity limitation which they impose on the flask type receptacles and of the risk of said receptacles breaking.
Accordingly, requirements can be assessed at about twice the present capacity.
Such a requirement should not pose any problem, in theory, inasmuch as the various dimensional characteristics governing the construction of the apparatus, receptacles and flasks, according to said French Patent, can conceivably be extrapolated.
It is, in effect, a question of a simple theoretical approach basically disregarding the energy used for raising the temperature of the sample to be treated.
One imperative way cf positively meeting the abovementioned requirements is to increase the diameter of the communication opening between the stack and the microwave application cavity. Having done this, the height of the stack should, as a result, be increased correspondingly, so that said stack always acts as a barrier preventing any microwave propagation into the ambient medium, thereby complying with the essential conditions imposed for the safety of the operating staff.
Now, it is conventionally known that, for microwave energies of 2.45 GHz, a limit of serviceable cross-section of the opening between application cavity and stack, is, theoretically, around 7 cm in diameter.
In actual fact this value is purely theoretical and the practical limit to be observed, when it has been decided to use receptacles or flasks having a serviceable capacity of 300 ml, is considerably below said value.
The fact of selecting a serviceable passage section which is compatible with receptacles of such capacity, results in a structure which is characterized by a permanent microwave leaking or conduction condition, without any protection of the environment or of the personnel in charge of the operation or control of these apparatus.
In an attempt to solve the problem raised by the permanent conduction of microwave energy through the stack, traps or doors could be used, whether or not in the form of half-shells, placed around the open upper end of the stack, as well as around that of the receptacle or flask.
Such a structure however, is cumbersome and not easy to use, and above all, it is expensive to design, to build, to acquire and to maintain.
In fact, such a technique would not either be entirely satisfactory if applied to the mineralization of inorganic, organic or organometallic compounds. Indeed, from the moment when the stack comprises a secondary cavity for applying the microwave energy, the height of the stack propagates the microwave energy which is responsible for a certain heating of the mineral compounds spreading over the whole serviceable height of the receptacle. Now, it is a known fact that the mineralization process induces the formation of vapors which, normally, have to be condensed in an intermediate part of the receptacle or flask, in order to create a reflux permitting a much more accurate control of the mineralization phase by limiting the effluents.
From the moment when the microwave energy can cause a rise in temperature of the already partly heated traces, vapors or elements of compounds, the condensation and reflux phase no longer occurs, and the compound or compounds are vaporized over the whole height of the stack.
In addition to the particularly important quantitative loss in the case of a mineralization reaction, it becomes necessary to introduce specific stopping or connecting means, in order to reduce the pollution of the surrounding medium and the losses.
In other words, although the structural proposition described in the French application is a considerable improvement over the prior art, it still is not absolutely satisfactory, because of the drawbacks mentioned hereinabove.